World powers held a fourth round of high-level talks Friday aimed at bringing the United States back into the nuclear deal with Iran, with both sides signaling a willingness to work out the major stumbling blocks.

The talks began in Austria in early April. Russian delegate Mikhail Ulyanov tweeted following Friday’s meeting that “the participants agreed on the need to intensify the process.”

“The delegations seem to be ready to stay in Vienna as long as necessary to achieve the goal,” he wrote.

The U.S. pulled out of the landmark 2015 deal in 2018 after then-President Donald Trump said the pact needed to be renegotiated. The deal had promised Iran economic incentives in exchange for curbs on its nuclear program, and the Trump administration reimposed heavy sanctions on the Islamic republic in an unsuccessful attempt to bring Tehran into new talks.

Iran reacted by steadily increasing its violations of the deal by enriching uranium to a greater purity than permitted, stockpiling more enriched uranium than allowed and using more advanced centrifuges, among other moves to try and pressure the powers remaining in the deal — Germany, France, Britain, Russia and China — for economic relief.

U.S. President Joe Biden says he wants to rejoin the deal, known as the Joint Comprehensive Plan of Action, or JCPOA, but that Iran needs to return to compliance.

Speaking to reporters at the White House on Friday, Biden said he believed the Iranians were approaching the talks seriously.

“But how serious and what they’re prepared to do is a different story,” Biden said. “We’re still talking.”

The pact is meant to prevent Iran from obtaining a nuclear bomb, something the country insists it does not want to do, and the government in Tehran has said it is prepared to reverse all of its violations but that Washington must remove all sanctions imposed under Trump.

Still unresolved is what Iran’s return to compliance would look like. Delegates to the Vienna talks concede, for example, that Iranian nuclear scientists cannot unlearn the knowledge they acquired in the last three years, but it is not clear whether Iran’s new centrifuges would need to be destroyed, mothballed and locked away, or simply taken offline.

Because the U.S. is currently out of the deal, there were no American representatives at the talks. Diplomats involved are shuttling between the Iranian side and a delegation from Washington elsewhere in Vienna.

Iran’s delegate to the talks, Deputy Foreign Minister Abbas Araghchi, told Iranian state television after the meeting that his impression was that all sides were committed to finding a solution.

“The reports that are being conveyed to us from Americans is that they are also serious about returning to JCPOA. So far, they have announced that they are ready to lift most of their sanctions, but we do not think it is enough,” Araghchi said.

“That is why the negotiations will continue until we reach all our demands in this regard,” he added. “If our demands are met, Iran will be quite serious about returning to its obligations in the full implementation of JCPOA.”

Between the high-level meetings in Vienna of the so-called Joint Commission, expert groups have been meeting to try and come up with solutions to the outstanding issues.

Alain Matton, a spokesperson for the EU delegation, which is chairing the meetings, said the expert discussions will continue in the days ahead.

“And the EU as a coordinator and facilitator of the JCPOA talks will continue with separate talks with all participants and with the U.S.,” Matton told reporters. “The participants are continuing with discussions, which are held on various levels and which have as their objective the full and effective implementation of the deal by all sides and the U.S. return to the JCPOA.”

Ahead of the talks, a senior U.S. official, speaking on condition of anonymity to discuss the U.S. position, said Washington has laid out the concessions it’s prepared to make and that success or failure now depends on Iran making the political decision to accept those concessions and to return to compliance with the accord.

The official said it remains possible to reach an agreement before Iran’s June presidential election, which some believe are a complicating factor in the discussions.

Heading into the talks, Ulyanov tweeted that he saw positive signs from both sides.

“The head of the Iranian delegation is cautious in his assessment of the current state of affairs at the Vienna talks (very similar to assessments of the US colleagues),” he tweeted. “But both #Iran and #US refrain from pessimistic conclusions. This seems to be not a bad sign.”